X-ray tomography device

ABSTRACT

An X-ray tomography device for providing a 3D tomography image of a sample comprising a X-ray source, a cell, a photon detector and a processing unit. The X-ray source is monochromatic and has a photon beam solid angle higher than 0.1 degree. The processing unit computes the 3D tomography image on the basis of acquired images corresponding to a plurality of cell angles.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/EP2012/060439, filed Jun. 1, 2012, which claims priority from U.S.Provisional Patent Application No. 61/492,268, filed Jun. 1, 2011, andU.S. Provisional Patent Application No. 61/492,272, filed Jun. 1, 2011,said applications being hereby incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The present invention concerns an X-ray tomography device.

BACKGROUND OF THE INVENTION

The present invention concerns an X-ray tomography device adapted topetrophysics application, such as to study the flow of fluids into aporous medium. For example, the aim is to study the multiphase flow of amix of two or three fluids inside a porous medium: a mix of any two ofwater, gas and oil or the three of them.

The known X-ray tomography systems are adapted to study the morphologyof rock pores, to identify the minerals comprised into the rock sample(the porous medium) or the topology of various fluid phases present inthe rock sample under static (ie non flowing) conditions.

Because of use of polychromatic X-ray source, these systems developapproximative results due to non linear absorption and therefore imageartefacts. The quality of images is therefore strongly impacted,especially with respect to the identification of materials (fluid orrock). The laboratory sources used in these systems have a very lowphoton flux that requires a very long recording time for the acquisitionof high resolution images. These systems thus do not provide anacquisition time compatible with the study of multiphase flow in porousmedia. These systems also use image reconstruction algorithms that mustdeal with large volumes of data to calculate only one 3D tomographyimage. Moreover the strong diverging angle of polychromatic X-raymicrotomographs introduces artifacts in the 3D image reconstructionsresulting from compromises in the complex reconstruction process in avery diverging geometry. These systems are unable to provide rapidly 3Dtomography images for generating a movie of fluid transport within theporous medium sample.

Consequently, these devices are only able to provide staticcharacteristic values inside the porous medium, such as irreduciblewater saturation or residual oil saturation. They are unable tovisualise the flow of a fluid or the flow of a plurality of fluidsinside the porous medium.

Synchrotron X-ray sources provide enough photon flux.

But, these devices provide a parallel photon beam having a very smallfocus spot size, varying about a few mm², that is incompatible with alarge field of view needed to observe macroscopic flow of fluids insidea porous medium and especially in realistic porous media wheredispersion, anisotropy, viscous fingering requires to be able to recordthe whole sample view. Additionally, these devices have huge size, arevery expensive and they are for scientific use only. It is difficult tohave access to such instrument for analysis of a petroleum porous mediumwhere experimental time may require waiting for several weeks up toseveral months.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide an X-ray tomographydevice that can be used to analyse the flow of fluids inside a porousmedium, such as a rock sample of a geological formation.

To this effect, the X-ray tomography device according to the inventionis adapted for providing a 3D tomography image of a sample, and itcomprises:

a X-ray source emitting a photon beam in the direction of a beam axis,said X-ray source being a near monochromatic source and said photon beamhaving a solid angle higher than 0.1 degree around said beam axis,a cell adapted to include a porous sample to be imaged, said cell beingsituated inside the photon beam and being able to rotate around a cellaxis that is substantially perpendicular to the beam axis, and beingadapted to enable the porous sample to be flooded by at least one fluid,a photon detector receiving a transmitted photon beam that istransmitted through said cell, said photon detector providing at leastone acquired image for each angle of a plurality of cell angles, andsaid acquired images being taken during a length of time lower than tenminutes, anda processing unit that computes the tomography image on the basis of theacquired images corresponding to the plurality of cell angles.

Thanks to these features, the X-ray tomography device is able to havesimultaneously, a high level of photons and a large field of view.

It is also able to have a very high level of photons and a small fieldof view permitting to work in stitching mode or local tomography mode

The volume analysed can be imaged during a length of time lower than tenminutes, which is very competitive with what is achieved with a 3^(rd)generation synchrotron,

It is therefore possible to get a plurality of 3D tomography imagesshowing a movie of the flow of fluids inside the porous medium of thesample. Moreover, it may possible to scan the whole volume and toidentify areas of fluid fluctuations before to zoom in to reach the bestresolution.

In various embodiments of the X-ray tomography device, one and/or otherof the following features may optionally be incorporated.

According to an aspect, the monochromatic and highly brilliant X-raysource is a compact light source using a collision between a laser beamand an opposing electron beam.

According to an aspect, the length of time for the volume analysis islower than one minute.

According to an aspect, the processing unit is computing the tomographyimage during a time period lower than the length of time of used forproducing the acquired images corresponding to the plurality of cellangles.

According to an aspect, the cell has a size comprised in the range of0.3 cm to 20 cm, and preferably in the range of 0.6 cm to 10 cm.

According to an aspect, the cell is made of a material in a listcomprising the beryllium, beryllium alloys, and a carbon-carboncomposite.

According to an aspect, the cell comprises means for heating the sampleto a temperature higher than 650° Celsius and means for pressuring thecell to a pressure higher than 1000 bars,

According to an aspect, the photon detector comprises a CCD of at leastten megapixels.

According to an aspect, the X-ray tomography device further comprises agrating based interferometer situated between the cell and the photondetector.

According to an aspect, the X-ray tomography device further comprises amicroscope situated between the cell and the photon detector.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be apparent from thefollowing detailed description of one of its embodiments given by way ofnon-limiting example, with reference to the accompanying drawings. Inthe drawings:

FIG. 1 is a schematic view of a X-ray tomography device according to theinvention, and

FIG. 2 is an example of a 3D tomography image provided by the device ofFIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

In the various figures, the same reference numbers indicate identical orsimilar elements. The direction Z is a vertical direction. A direction Xor Y is a horizontal or lateral direction. These are indications for theunderstanding of the invention.

The X-ray tomography device 1 shown on the FIG. 1 comprises:

a X-ray source 2 emitting a photon beam PB in the direction of a beamaxis BA,a cell 3 comprising a porous sample 10 to be imaged,a photon detector 4 receiving a transmitted photon beam TPB that istransmitted through said cell 3, anda processing unit 5 computing the 3D tomography image on the basis ofthe acquired images provided by the photon detector 4.

The X-ray source 2 is preferably a monochromatic source, so that thecell is illuminated with a high level of brilliance by an X-ray beam ofsmall diverging angle. The polychromatic sources spread their energyinto a wide frequency bandwidth. It is possible to produce a naturalmonochromatic flux of photons or to filter the photon beam PB to obtaina quasi-monochromatic photon beam. However, this decreases a lot thephoton flux. The monochromatic source concentrates the energy on a verynarrow frequency bandwidth. The length of time needed by a detector foracquiring an image is then very low, and then it is compatible withmultiphase flow tracking.

The photon beam PB generated by said X-ray source 2 is a diverging conebeam having a solid angle SA that is wide, and for example higher than0.1 degree or a few mrad around the beam axis BA. It is possible toilluminate a complete cell having a size of 10 cm at a distance from theX-ray source 2 that is a small distance, for example lower than 25 m,and preferably lower than 10 m. The solid angle SA may be higher than0.5 degree.

Preferably, the X-ray source is able to emit a photon beam having a highlevel of energy, for example comprised between 10 and 200 KeV. Thephoton flux may be higher than 10⁸ photons/s near the photon detector 4,and preferably higher than 10¹¹ photons/s. The device is then able toimage thick cells and thick samples (between 0.3 cm and 10 cm). TheX-ray source may have a tuneable X-ray energy level.

For example, the X-ray source 2 may be a compact photon source usingcollision between a laser beam and an opposing electron beam. Such X-raysource 2 preferentially uses Inverse Compton Effect (Thomson scattering)to generate a natural monochromatic photon beam PB having a high levelof energy. The main advantage of such X-ray sources is that they arevery compact compared to classical synchrotron devices. Known Table-topsynchrotron device using such physical properties are the “Compact LightSource” (CLS) from Lyncean Technologies Inc., but filtering verybrilliant polychromatic flux such “Mirrorcle” from Photon Production Labmay produce a quite similar result.

The X-ray source 2 may be tuneable according to the energy level(brilliance) so as to proceed to various experiments above the poroussample.

The cell 3 is situated inside the photon beam PB. The cell position canbe controlled via a rotation mean 8 (Z rotation) and a translation mean9 (XYZ translations).

Thanks to the rotation mean 8, the cell 3 can be rotated around a cellaxis CA substantially parallel to axis Z and perpendicular to the Xaxis, the beam axis BA on FIG. 1. The cell 3 is rotated of a cell anglearound the cell axis CA. The detector 4 can then provide images from thecell (sample) from various view angles and the processing unit 5 cancompute a 3D tomography image of the sample.

Thanks to the translation mean 9, the cell 3 can be positioned insidethe photon beam PB.

The cell 3 can be placed or positioned between a first distance from thesource 2 and a second distance from the source 2. The first distance maybe short and the cell 3 is close to the X-ray source 2 (see position P1on FIG. 1). This configuration optimizes the maximal flux in highresolution (stitching mode or local tomography). The second distance ismuch higher than the first distance, the cell 3 being away from theX-ray source 2 In this configuration, it is possible to illuminate thewhole region of interest permitting to easily switch from a globaltomography mode to local tomography based on observed changes induced bythe multiphase flow. The acquisition time in this last configuration isless performing than the first one but it permits to analyse the samplein interactive mode

For example, the cylindrical rock sample contained inside the cell 3 hasa size comprised in the range of 0.3 cm to 10 cm. The size is preferablyin the range 0.6 cm to 3 cm in diameter and in the range of 2 cm to 10cm in length. The size of the rock sample is chosen big enough to studymultiphase transport properties at a scale representative of macroscopictransport properties in the said rock and small enough to enable highresolution tomography of the sample in a length of time that allowsimaging the whole sample in less than ten minutes: acquiring the imagesfrom the plurality of cell angles within said length of time.

The cell 3 is for example a tube extending along the cell axis CA, saidtube receiving the sample of porous medium. The cell 3 comprises aninput conduct 6 that input the fluid to the cell 3 and an output conduct7 that outputs the fluid from the cell. The cell is adapted to becrossed by the fluid.

The X-ray tomography device 1 also comprises hydraulic devices toprovide the fluid to the input conduct and to get back this fluid fromthe output conduct. These hydraulic devices can also add physicalconditions to the fluid: temperature, pressure. To this end, thesehydraulic devices include a thermal regulator, and a pressure regulator.The sample 10 inside the cell 3 can be tested according to the physicalconditions of the geologic formation.

The thermal regulator can heat the sample up to a temperature of 650°Celsius.

The pressure regulator can pressurize the sample up to a pressure of1000 bars.

The cell 3 is a sort of Hassler cell meeting the requirements of X-raytomography imaging. The cell 3 is adapted to enable the porous sample 10to be flooded by one or several fluids under controlled pressure andtemperature conditions.

The cell 3 is made of a material that is transparent to the X-ray photonbeam. Advantageously, it is made of beryllium, or beryllium alloy suchberyllium aluminium alloy, or a carbon-carbon composite.

The photon detector 4 can be tuned to have a sensitivity correspondingto the sample and fluids. Small variations of fluid densities can betherefore detected. Oil and water can be distinguished in the acquiredimages provided by the photon detector 4 using very fast classicalabsorption mode, or phase mode or dark field mode.

The photon detector 4 is providing at least one image for each angle ofa plurality of cell angles. All these acquired images are taken during alength of time lower than ten minutes for the whole volume to analyse.It is assumed that the state of the sample does not change much duringthis length of time: the fluid movements inside the porous medium remainvery small. All the acquired images from various cell angles are thensupposed to represent a unique state of the sample.

Advantageously, the length of time is lower than one minute. The imagesrepresent more precisely a unique state of the sample, and thetomography device is acquiring images in real time and stores all theseimages for the processing unit 5.

The photon detector 4 can be a flat panel, or an X-ray CCD(Charge-Coupled Device) or a CMOS. The photon detector 4 has a highresolution. It is for example a CCD having at least ten megapixels. Theacquired images are enough accurate to visualise at the same time(simultaneously) the complete field of view of the sample or very smalldetails inside the sample thanks to a stitching mode or local tomographyprocess. In this way several ways are possible to scan the sample, andthe acquired image can be taken in a very short length of time and theacquired image is enough exposed to photon flux to show small detailsand small variations of densities.

The processing unit 5 is computing the 3D tomography image on the basisof the acquired images corresponding to the plurality of cell angles.Such reconstruction method is known and efficient (fast and providing avery good image quality) benefiting from the quasi parallelapproximation. Examples of reconstruction methods can be found in thefollowing document:

A. C. Kak and Malcolm Slaney, Principles of Computerized TomographicImaging, IEEE Press, 1988.

In the present invention, the processing unit 5 may comprise parallelcomputing means so that the 3D tomography image can be computed during avery short time period. This high performance for reconstruction timeand imaging are mainly due to the quasi parallel beam geometry. The timeperiod can be lower than the length of time for acquiring the imagesfrom various cell angles of the sample. The X-ray tomography device istherefore generating real time 3D tomography images, and can visualize areal time movie showing the fluids movements inside the porous medium.

The tomography device 1 may comprise a microscope to obtain high(accurate) resolutions. In that case, the resolution may reach 200 nm ofvoxel size which is the theoretical limit of microscopes due to Rayleighcriterion.

The tomography device 1 may also comprise a grating basedinterferometer, situated between the cell 3 and the microscope or thephoton detector 4. Such gratings improve the contrast of the acquiredimages by adding absorption contrast image, phase contrast image anddark field contrast image: materials having similar densities can bedistinguished on the acquired images by photon detector 4. In that case,the same resolution than obtained only by the microscope can beobtained.

The gratings, the microscope and the detector 4 compose an opticalstation of the X-ray tomography device 1.

The FIG. 2 is showing an example of a projection of 3D image 20 providedby the X-ray tomography device 1 of the invention. The 3D tomographyimage comprises various gray levels or various colours, eachrepresenting a constituent of the sample. The reference 21 representsthe porous medium. The reference 22 represents a first fluid having afirst density. The reference 23 represents a second fluid having asecond density.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments may be within the claims. Although the presentinvention has been described with reference to particular embodiments,workers skilled in the art will recognize that changes may be made inform and detail without departing from the spirit and scope of theinvention.

Various modifications to the invention may be apparent to one of skillin the art upon reading this disclosure. For example, persons ofordinary skill in the relevant art will recognize that the variousfeatures described for the different embodiments of the invention can besuitably combined, un-combined, and re-combined with other features,alone, or in different combinations, within the spirit of the invention.Likewise, the various features described above should all be regarded asexample embodiments, rather than limitations to the scope or spirit ofthe invention. Therefore, the above is not contemplated to limit thescope of the present invention.

1. An X-ray tomography device for providing a 3D tomography image of asample, said device comprising: a X-ray source (2) emitting a photonbeam in the direction of a beam axis, said X-ray source being a nearmonochromatic source and said photon beam having a solid angle higherthan 0.1 degree around said beam axis, a cell (3) adapted to include aporous sample to be imaged, said cell being situated inside the photonbeam and being able to rotate around a cell axis that is substantiallyperpendicular to the beam axis, and being adapted to enable the poroussample to be flooded by at least one fluid, a photon detector (4)receiving a transmitted photon beam that is transmitted through saidcell, said photon detector providing at least one acquired image foreach angle of a plurality of cell angles, and said acquired images beingtaken during a length of time lower than ten minutes, and a processingunit (5) that computes the tomography image on the basis of the acquiredimages corresponding to the plurality of cell angles.
 2. The X-raytomography device according to claim 1, wherein the monochromatic andhighly brilliant X-ray source is a compact light source using acollision between a laser beam and an opposing electron beam.
 3. TheX-ray tomography device according to claim 1, wherein the length of timefor the volume analysis is lower than one minute.
 4. The X-raytomography device according to claim 1, wherein the processing unit iscomputing the tomography image during a time period lower than thelength of time of used for producing the acquired images correspondingto the plurality of cell angles.
 5. The X-ray tomography deviceaccording to claim 1, wherein the cell has a size comprised in the rangeof 0.3 cm to 20 cm, and preferably in the range of 0.6 cm to 10 cm. 6.The X-ray tomography device according to claim 1, wherein the cell ismade of a material in a list comprising the beryllium, beryllium alloys,and a carbon-carbon composite.
 7. The X-ray tomography device accordingto claim 1, wherein the cell comprises means for heating the sample to atemperature higher than 650° Celsius and means for pressuring the cellto a pressure higher than 1000 bars,
 8. The X-ray tomography deviceaccording to claim 1, wherein the photon detector comprises a CCD of atleast ten megapixels.
 9. The X-ray tomography device according to claim1, further comprising a grating based interferometer situated betweenthe cell and the photon detector.
 10. The X-ray tomography deviceaccording to claim 1, further comprising a microscope situated betweenthe cell and the photon detector.